Mobile stations that communicate with wireless networks are frequently required to operate in different frequency bands. Different frequency bands may be used, for example, in different geographical regions, for different wireless providers, and for different wireless network systems. Mobile stations therefore often require an internal antenna responsive to multiple frequency bands including a lower frequency band, such as GSM850/900 band (824 to 960 MHz), and a higher frequency band, such as DCS (1710 to 1850 MHz), PCS (1850 to 1990 MHz) and UMTS (1920 to 2170 MHz).
Among the various choices for internal antennas in mobile stations, planar inverted-F antenna (PIFA) has been often adopted in practical application. Relative to other internal antennas, the PIFA is generally lightweight, easy to adapt and integrate into a device, and has moderate range of bandwidth. Conventional designs of PIFA for dual-band operation are disclosed in Japanese Laid-open Patent Publication No. 2006-295876, International Publication Pamphlet No. WO 2004/015810 A1, and International Publication Pamphlet WO 2004/038857 A1, for example.